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    <title>Document</title>
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<body>

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<script>
    var arr = [];
    var newArr = [];
    var average = 0;
    arr.length = 30;
    for (var i = 1; i <= arr.length; i++) {
        arr[i - 1] = 2 * i;

        if (i % 5 !== 0) {
            average += arr[i - 1];
        } else {
            newArr.push(average / 5);
            average = 0;
        }
    }
    console.log(arr);
    console.log(newArr);
</script>

</html>